The recent widespread use of radio frequency (RF) transmitters, such as in citizen band (CB) radios, for example, has created the need for an efficient and economical test instrument which can measure operational characteristics such as the standing wave ratio (SWR) of the transmitter, antenna combination; the output power from the transmitter; and the radiated power from the antenna.
The standing wave ratio measurement compares the value of the RF signal fed toward the antenna, to the value of the RF energy reflected therefrom. Heretofore, this measurement has normally been accomplished by means which include a wire electro-magnetically coupled to a segment of the transmission cable between the transmitter and the antenna, and means for sequentially measuring the relative percentage of the energy thereon which is traveling in the forward and reverse directions, respectively. From a functional point of view, this arrangement has proven satisfactory; however, it does not lend itself to economy in manufacture which may be realized from printed circuit designs.
The measurement of transmitter output power has normally been implemented by means for applying a rectified sample of the output power from the transmitter to a parallel combination of a potentiometer and a capacitor, with the wiper of the potentiometer being electrically connected to a meter, see FIG. 4. Such an arrangement loads the transmitter and prevents calibration for maximum allowable power. This is an important factor in applications such as CB radios where power is limited by government regulation.